1. Field of the Invention
The present invention relates to a power converter, and more specifically relates to a switching power converter.
2. Description of Related Art
Power converters have been widely used for providing regulated outputs. For safety, the power converter must provide galvanic isolation between its primary side and secondary side. A transformer is usually equipped for providing the isolation and energy transfer. FIG. 1 shows a traditional power converter having a transformer 10. The energy is stored inside the transformer 10 when a switch 20 is turned on. The energy shall be discharged to the output of the power converter when the switch 20 is turned off. A resistor 21 is connected in series with the switch 20 for sensing the switching current of the transformer 10 and for generating a current signal VS for the switching control. A controller 25 is coupled to the transformer 10 and the resistor 21 for generating a control signal VG to control the on/off of the switch 20 and to regulate the output of the power converter. The transformer 10 includes a primary winding NP, a secondary winding NS, and an auxiliary winding NA. Once the switch 20 is turned off, the auxiliary winding NAshall generate a reflected voltage VF correlated to the output voltage VO. Therefore, the reflected voltage VF can be utilized to feedback the output voltage VO. A reflected voltage control technique has been disclosed in U.S. Pat. No. 4,302,803 “Rectifier-Converter Power Supply with Multi-Channel Flyback Inverter”. However, the disadvantage of the aforementioned reference is that the reflected voltage cannot be accurately measured, particularly at the light load condition.
FIG. 2 shows a plurality of voltage waveforms of the power converter at the light load. The discharge time TDS of the transformer 10 is given by:
                              T          DS                =                              (                                          V                IN                                                              V                  O                                +                                  V                  D                                                      )                    ×                                    W              NS                                      W              NP                                ×                      T            ON                                              (        1        )            where VIN is the input voltage of the power converter, WNP and WNS respectively are the winding turns of the primary winding NP and the secondary winding NS of the transformer 10, VD is a forward voltage drop of the rectifier 15, and TON is an on-time of the control signal VG. The reflected voltage VF is connected to the controller 25 through a resistor 22 in FIG. 1. A voltage VDET is therefore produced in the controller 25 for the reflected voltage detection. However, a parasitic capacitor 23 and the resistor 22 cause a low pass filtering to the reflected voltage VF. Apart from the fact that the on-time TON of the control signal VG and the discharge time TDS of the reflected voltage VF are short at light load, the waveform distortion of the voltage VDET is shown in FIG. 2. A lower reflected voltage is therefore detected. The main objective of the present invention is for overcoming the aforementioned drawback.